Mechanically Pre-biased Shadow Mask and Method of Formation

ABSTRACT

Shadow masks comprising a multi-layer membrane having a mechanical pre-bias that compensates the effect of gravity on the membrane are disclosed. A shadow mask in accordance with the present disclosure includes a membrane that is patterned with a desired pattern of apertures. The layers of the membrane are selected such that their residual stresses collectively give rise to a stress gradient that is directed normal to the plane of the membrane such that the stress gradient mitigates gravity-induced sag. In some embodiments, the membrane includes a layer pair having internal stresses that are of opposite signs to effect a tendency to bulge outward from the plane of the membrane prior to its release from the substrate. An exemplary membrane includes a layer pair comprising a layer of stoichiometric silicon dioxide that is under residual compressive stress and a layer of stoichiometric silicon nitride that is under residual tensile stress.

STATEMENT OF RELATED CASES

This case claims priority to U.S. Provisional Patent Application Ser.No. 62/492,659 filed on May 1, 2017 (Attorney Docket: 6494-215PR1),which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to material deposition in general, and,more particularly, to direct patterning of a material layer on asubstrate via deposition of the material onto the substrate through ashadow mask.

BACKGROUND

Semiconductor fabrication requires the formation of one or patternedlayers of material on the surface of a substrate. The most commonapproach for forming a material pattern is to deposit a full-surfacelayer of the material over the entire surface of the substrate and thenremove the material everywhere except where it is desired. This iscommonly referred to as “subtractive patterning.”

Unwanted material is removed in a multi-step process in which a layer ofphotoresist is formed over the material layer and illuminated with apattern of light that is based on the desired material pattern. After ithas been exposed, the photoresist covering the unwanted material isdissolved in a strongly basic developer solution, which leaves behind aphotoresist mask that covers the material to remain on the substrate. Anetchant is then used to remove the exposed material, thus patterning thematerial pattern as desired. Afterward, the photoresist mask must beremoved and the substrate must be thoroughly cleaned to ensure noresidue of photoresist or etch product remains on any of its surfaces.

During a subtractive patterning process, everything on the substrate(i.e., any previously defined structures and materials, etc.) is exposedto harsh chemicals, including the photoresist developer solution, theetchant used to pattern the material layer, and the chemicals used toclean the substrate. Unfortunately, many materials, such as organic andbiological materials, cannot survive exposure to one or more of thesechemicals. As a result, subtractive patterning cannot be used for such“sensitive materials” or to pattern any material layer formed subsequentto deposition of a sensitive material on a substrate. For suchoperations, therefore, a direct patterning process must be used.

A direct patterning process forms a desired pattern of material as it isdeposited, thereby avoiding the need for post-deposition treatments andthe harsh chemicals they normally involve. One such direct-patterningprocess is shadow-mask deposition, which is analogous to stencil-basedprinting techniques, such as stencil painting, silk screen printing, andthe like.

During shadow-mask deposition, vapor molecules of the material aregenerated such that they flow from a source toward the substratesurface. The vapor molecules can be generated via any of a variety ofprocesses, such as evaporation, sputtering, and the like. A thin layerof structural material having a pattern of apertures (i.e., openings)that matches the pattern desired for the deposited material (referred toas a “shadow mask”) is positioned just in front of (but typically not incontact with) the substrate surface. When the flow of material reachesthe shadow mask, the passage of material to the substrate is blockedeverywhere except at the apertures. As a result, the material layer isdirectly patterned during its deposition on the substrate and noadditional post-deposition processing is required.

Historically, shadow-mask deposition has been used in semiconductorfabrication to define patterns of relatively large (>50 micron)features, such as wire-bond pads, etc. A typical shadow mask used insuch applications includes a thin, patterned metal sheet held by anannular frame. While the minimum feature size and minimum separationbetween apertures for such shadow masks is quite large (typicallygreater than several tens of microns), such shadow masks are perfectlysuitable for defining large-feature-size, sparse patterns of materiallike wire-bond pad patterns.

More recently, it has become desirable to employ shadow-mask depositionin the formation of electronic devices based on organic materials, suchas organic light-emitting diodes (OLED), active-matrix OLED displays,organic solar cells, biological-material-based sensors, and the like. Inmany cases, much higher resolution and pattern density is required thancan be achieved with a conventional metal-layer-based shadow mask. As aresult, high-performance shadow masks have been developed that enablefeature sizes and pattern densities that are less than or equal to tenmicrons.

Such high-performance shadow masks typically have a very thin (<1micron) layer of structural material (e.g., silicon nitride, silicon,etc.) disposed on an annular frame formed from a semiconductor or glasshandle substrate. The apertures are formed in the thin structural layer,after which the center portion of the handle substrate is removed,leaving the central region of the structural layer as a patternededge-supported membrane.

Theoretically, during shadow-mask deposition, material deposits only onthe surface of the substrate in those regions located directly behindthe apertures. In practice, however, as the vaporized molecules travelfrom the source to the shadow mask, many vaporized molecules propagatealong directions that are not perfectly normal to the shadow mask andsubstrate. As a result, some vaporized molecules continue to travellaterally after passing through the shadow mask such that the resultantpatterned regions extend beyond the edges of the apertures. Themagnitude of this lateral spreading of the features (referred to as“feathering”) is a function of the separation distance between thesubstrate surface and the shadow mask, which is preferably very small—afew microns at most, as well as the orientation of the source relativeto the center of the shadow mask.

While feathering is not a critical issue when forming large, widelyspaced features (e.g., wire-bond pads, etc.), it can be highlyproblematic when forming small-feature, highly dense patterns. In fact,feathering has been a limiting factor for the minimum feature size andpattern density attainable using shadow-mask deposition.

Furthermore, many applications, such as high-resolution OLED displayfabrication, require dense patterns of small features (<10 microns) thatextend over a large area (greater than tens of centimeters). Thisrequires a shadow mask that is both very thin (to mitigate shadowing andenable fine feature definition) and very large (lateral extents of tensof centimeters). Unfortunately, thin, large-area membranes exhibitsignificant gravity-induced sag. For shadow masks of several inches indiameter, such sag can give rise to a significant variation (severalmicrons or more) in the separation distance between the substratesurface and the shadow mask over the area of the shadow mask. This, inturn, leads to increased feathering near the center of the shadow maskand an overall variation in the magnitude of the feathering that occursacross the area of the deposited pattern.

To mitigate the effect of gravity-induced shadow-mask sag in theprior-art, a mechanical grid of support material is sometimes placedunder the shadow mask membrane. Alternatively, regions of the shadowmask substrate can be left intact during its fabrication to providesupport over the area of the membrane. Unfortunately, these approachesgive rise to shadowing effects that lead to poor deposition uniformityover the surface area of the substrate.

The need for a shadow-mask capable of defining high-resolution directlydeposited layers without shadowing remains, as yet, unmet in the priorart.

SUMMARY

The present disclosure presents apparatus and methods that enable directpatterning of a material layer via shadow-mask deposition without someof the costs and disadvantages of the prior art. Embodiments of thepresent invention mitigate shadow mask sag due to gravity, therebyenabling directly patterned layers having smaller minimum features andhigher pattern density. Embodiments of the present invention areparticularly well suited for use in fabricating organic light-emittingdiodes (OLEDs), OLED displays, organic solar cells, and the like.

Embodiments in accordance with the present disclosure are shadow masksand shadow-mask-based deposition processes, where the shadow-maskincludes a multi-layer membrane formed on a handle substrate, and wherethe constituent layers of the membrane collectively give rise to astress gradient that is directed normal to the plane of the membrane.When arranged in a deposition system such that the membrane ishorizontal, he stress gradient effects a mechanical prebias thatcompensates for the effect of gravity on the membrane. As a result,gravity-induced sag can be mitigated.

The multi-layer membrane comprising a first layer of a first materialthat is characterized by a residual tensile stress and a second layer ofa second material characterized by a residual compressive stress. Thethicknesses and stresses of the layers are chosen such that the positionof the stress-neutral plane of the composite membrane is displaced fromthe middle of its thickness, thereby giving rise to a bending momentnormal to the membrane. In accordance with the present disclosure, thefirst and second layers are formed on a rigid handle substrate beforethe membrane is released from the substrate by forming a cavity in it.As a result, each of the first and second layer is characterized by aresidual stress induced in the layer during its formation on the rigidsubstrate.

Upon release of the multi-layer membrane from the handle substrate, itsconstituent layers attempt to relieve their internal strain such thatthe compressively stressed layer laterally expands while the tensilelystressed layer laterally contracts. As a result, the membrane ismechanically pre-biased to “bulge” to attain its lowest possiblestrain-energy state. By orienting the shadow mask such that themechanical pre-bias is in the direction opposite to that of gravity, sagof the membrane due its own weight is mitigated.

An illustrative embodiment includes a bilayer membrane in which thefirst layer is stoichiometric silicon nitride and the second layer isstoichiometric silicon dioxide, where the first layer has a thickness ofapproximately 50 nm and a residual tensile stress of approximately 1 GPaand the second layer has a thickness of approximately one micron and aresidual compressive stress of approximately 300 MPa. In someembodiments, at least one of the first and second layers has a differentthickness and/or residual stress.

In some embodiments, the membrane includes more than two layers.

In some embodiments, the first layer comprises a compressively stressedmaterial other than stoichiometric silicon dioxide. In some embodiments,the second layer comprises a tensilely stressed material other thanstoichiometric silicon nitride.

In some embodiments, a portion of the substrate is left intact in themembrane region such that the membrane includes a thin layer ofsubstrate material.

An embodiment of the present invention is an apparatus comprising ashadow mask that includes: a substrate that defines a first plane, thesubstrate including a cavity; and a composite layer that is disposed onthe substrate, the composite layer having a first region that includesan aperture pattern comprising at least one aperture, wherein thecomposite layer comprises a plurality of layers that includes a firstlayer having a first thickness and a first residual stress and a secondlayer having a second thickness and a second residual stress, whereinthe first region of the composite layer is disposed over the cavity anddefines a membrane; wherein the first thickness, second thickness, firstresidual stress, and second residual stress collectively give rise to afirst bending moment that is directed along a direction that issubstantially normal to the first plane.

Another embodiment of the present invention is a method for forming ashadow mask, the method comprising: providing a substrate that defines afirst plane; forming a composite layer on the substrate, the compositelayer including: a first layer comprising a first material, the firstlayer having a first thickness and a first residual stress; and a secondlayer comprising a second material, the second layer having a secondthickness and a second residual stress; defining an aperture pattern ina first region of the composite layer, wherein the aperture patternincludes at least one aperture, and wherein the at least one apertureextends completely through the composite layer; and forming a cavity inthe substrate, wherein the cavity is formed after the formation of thecomposite layer, and wherein the formation of the cavity releases thefirst region of the composite layer to define a membrane; wherein thefirst thickness, second thickness, first residual stress, and secondresidual stress are selected such that the membrane has a first bendingmoment that is directed along a direction that is substantially normalto the first plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic drawing of an illustrative embodiment of ashadow-mask deposition system in accordance with the present disclosure.

FIG. 2 depicts a schematic drawing of a cross-sectional view of anexemplary high-resolution shadow mask in accordance with the prior art.

FIG. 3 depicts a schematic drawing of a cross-sectional view of a shadowmask 108.

FIG. 4 depicts operations of a method for forming a mechanicallypre-biased shadow mask in accordance with the illustrative embodiment.

FIG. 5A depicts a schematic drawing of a cross-sectional view of nascentshadow mask 108 after the formation of composite layer 204 on surface206 and mask layer 402 on surface 222.

FIG. 5B depicts a schematic drawing of a cross-sectional view of nascentshadow mask 108 after the definition of apertures 126 and mask 224.

FIG. 5C depicts a schematic drawing of a cross-sectional view of nascentshadow mask 108 after the formation of cavity 212.

FIG. 5D depicts a schematic drawing of a cross-sectional view of nascentshadow mask 108 when oriented such that residual-stress-induced bendingmoment 218 and gravity-induced bending moment 408 are balanced.

DETAILED DESCRIPTION

FIG. 1 depicts a schematic drawing of an illustrative embodiment of ashadow-mask deposition system in accordance with the present disclosure.System 100 includes vacuum chamber 102, source 104, mask chuck 106,shadow mask 108, substrate chuck 110, and positioning system 112. System100 is a vertical deposition system for directly patterning an OLEDmaterial layer on substrate 118.

Vacuum chamber 102 is a conventional pressure vessel operative forproviding a low-pressure atmosphere that supports evaporation ofmaterial 114. It should be noted that vacuum chamber 102 can be astandalone unit, part of a cluster deposition system, or part of atrack-deposition system where multiple evaporation chambers are arrangedin linear chain. In some embodiments, vacuum chamber 102 includesseveral evaporation sources/shadow mask combinations that enableformation of different patterns of different materials.

Source 104 is crucible for vaporizing material 114 to create vapor plume116. In the depicted example, material 114 is an organic materialsuitable for use in an OLED and source 104 acts substantially as a pointsource for the vaporized material because the open area of its crucibleis significantly smaller than the area of substrate 118.

In some embodiments, source 104 is a linear evaporation source thatcomprises a plurality of nozzles arranged along a longitudinal axis suchthat the nozzles collectively emit a fan-shaped vapor plume of vaporizedatoms. In some embodiments, positioning system 112 moves the linearsource along a direction that is unaligned with its longitudinal axis inthe x-y plane to improve the uniformity of the deposited material onsubstrate 118. In some embodiments, this path is a line that issubstantially orthogonal to both the linear arrangement of nozzles andnormal axis 124. In some of embodiments, the linear source is movedalong a non-linear path in the x-y plane.

In some embodiments, source 104 includes a two-dimensional arrangementof nozzles, each of which emits a conically shaped vapor plume such thatthe plurality of nozzles collectively provides a flow of vaporized atomsthat is substantially uniform over the area of the substrate surface. Insome embodiments, positioning system 112 moves the two-dimensionalarrangement of nozzles to facilitate deposition uniformity. In someembodiments, the two-dimensional arrangement of nozzles is rotatedin-plane to facilitate deposition uniformity.

In some embodiments, source 104 is a two-dimensional planar source thatincludes a layer of material 114 distributed across its top surface. Thesource is arranged such that this top surface is parallel to and facingsubstrate 118. When heated, material 114 vaporizes uniformly across theplane. Exemplary planar evaporation sources suitable for use inembodiments in accordance with the present disclosure are disclosed byTung, et al., in “OLED Fabrication by Using a Novel Planar EvaporationTechnique,” Int. J. of Photo energy, Vol. 2014(18), pp. 1-8 (2014),which is incorporated herein by reference.

In some embodiments, positioning system 112 imparts a relative motionbetween source 104 and the combination of substrate 118 and shadow mask108 to improve the uniformity with which material 114 deposits over thetwo-dimensional surface area of substrate 118. The relative motion isimparted by moving at least one of the substrate/mask combination andthe source. In some embodiments, a collimator (not shown) is insertedbetween source 104 and shadow mask 108 such that only vaporized atoms ofmaterial 114 traveling along directions aligned, or nearly aligned, withvertical axis 124. The inclusion of such a collimator can improve theuniformity with which material 114 deposits over the two-dimensionalsurface area of substrate 118. Collimators suitable for use inembodiments in accordance with the present disclosure are described inU.S. Patent Publication No.: 2017/0342542, which is incorporated hereinby reference.

Mask chuck 106 is a mechanical clamp that locates shadow mask 108between source 104 and substrate 118.

Shadow mask 108 is an element that includes a layer of structuralmaterial having a plurality of apertures whose size and arrangement arebased on the desired deposition pattern for material 114. The surface ofshadow mask 108 proximal to substrate 118 defines plane 120. Shadow mask108 is described in more detail below and with respect to FIGS. 3-5.

Substrate chuck 110 is a platen for securing substrate 118 such that thesubstrate is as flat as possible.

Substrate 118 is a glass substrate suitable for supporting theplanar-processing-based fabrication of an OLED display. In someembodiment, substrate 118 is a different conventional substrate, such asa semiconductor wafer (e.g., silicon, gallium arsenide, indiumphosphide, etc.), composite substrate, etc., that is suitable for planarprocessing. The surface of substrate 118 that is proximal to shadow mask108 defines plane 122.

Positioning system 112 is a multi-dimensional alignment system forcontrolling the relative positions of substrate 118, source 104, andshadow mask 108. In operation, positioning system 112 aligns the shadowmask and substrate such that they are separated by separation, s,(typically a few tens or hundreds of microns) along vertical axis 124,planes 120 and 122 are substantially parallel, and the apertures of theshadow mask are aligned with their respective deposition sites onsubstrate 118. In the depicted example, vertical axis 124 is alignedwith the direction of gravity and shadow mask 108 and substrate 118 areheld such that each of planes 120 and 122 is substantially orthogonalwith vertical axis 124 (i.e., θ=90°). In some embodiments, shadow mask108 and substrate 118 are held such that each of planes 120 and 122 isat an angle, θ, other than 90° to with respect to the direction ofgravity.

When heated, source 104 melts material 114 to generate vapor plume 116.As discussed above, vapor plume 116 includes vaporized atoms havingpropagation directions that span a relatively large angular range. As aresult, vaporized atoms travel some lateral distance after passingthrough the apertures of the shadow mask—referred to as “feathering.”Feathering gives rise to deposition of material 114 in unintendedregions of the substrate, which causes enlargement of the depositedfeatures and/or undesirable intermixing of different materials depositedin different depositions. Feathering, therefore, can limit the minimumfeature size of a deposited pattern, as well as pattern density.

The amount of feathering that occurs is determined by the lateral androtation alignments between planes 120 and 122, the separation, s,between them, and the range of propagation angles of the vaporized atomsincident on the shadow mask.

As discussed above, while prior-art shadow masks are theoretically flat,in practice they sag significantly due to the fact that most of theshadow mask is mechanically unsupported. As a result, a cross-sectionthrough any diameter of a prior-art shadow-mask membrane assumes asubstantially catenary shape (approximately a hyperbolic cosinefunction) such that its top surface is non-planar. For prior-art shadowmasks, therefore, the separation between the shadow mask and thetarget-substrate surface on which deposition is desired is non-uniform,which exacerbates the problems associated with feathering.

FIG. 2 depicts a schematic drawing of a cross-sectional view of anexemplary high-resolution shadow mask in accordance with the prior art.Shadow mask 200 is analogous to high-resolution shadow masks disclosedin U.S. Pat. No. 9,142,779, which is incorporated herein by reference.

Shadow mask 200 comprises layer 204, which is a layer of structuralmaterial disposed on the top surface of handle substrate 202. Thecentral region of the structural layer is a membrane (i.e., membrane206) in which apertures 208 are formed to enable passage of vaporizedmolecules through the shadow mask to deposit in a desired pattern onsurface 218 of target substrate 216. The membrane is defined by theformation of cavity 210 in handle substrate 202. In many applications,to be useful for forming large-area material patterns, the width (ordiameter) of cavity 210 must be at least several tens of centimeters.

Furthermore, in order to enable direct patterning of a dense pattern offeatures smaller than 10 microns, layer 204 is preferably verythin—typically, having a thickness equal to one micron or less.

It is preferred that the structural material of layer 204 ischaracterized by a large residual tensile stress to mitigategravity-induced sag of the membrane upon release from substrate 202.However, the magnitude of the residual stress in layer 204 must be belowthe fracture stress of membrane 206. For the purposes of thisSpecification, including the appended claims, the term “fracture stress”is defined as the magnitude of residual stress in a layer at which amembrane of a given size made from that layer will fracture upon itsrelease from its underlying substrate. One skilled in the art willrecognize that the fracture stress for membrane 206 decreases as itslateral dimensions increase. In other words, for a given level ofresidual stress in layer 204, there is a maximum lateral dimension(e.g., diameter or width) with which membrane 206 can be formed. If thislateral dimension is exceeded, the fracture stress for the material isexceeded and the membrane will fracture upon its release from substrate202.

To satisfy these conflicting requirements, in the prior art, layer 204is formed as a one-micron-thick layer of silicon nitride having anincreased silicon content (i.e., silicon-rich silicon nitride). Thesilicon content is chosen to reduce the magnitude of its residualtensile stress to approximately 300 MPa from the residual tensile stressof approximately 1 GPa for stoichiometric silicon nitride (i.e., Si₃N₄).

While it would be preferable to use stoichiometric silicon nitride forlayer 204 to realize a more taut membrane that would exhibit lessgravity-induced sag, the high residual stress of Si₃N₄ limit the size ofsuch a membrane to lateral dimensions of less than a few millimeters. Asa consequence of the reduced tensile stress in layer 204, the separationbetween the top surface 212 of mask 200 and surface 218 of targetsubstrate 216 is a function of radial distance from the center of themembrane non-uniform, s(x). The non-uniformity of this separationexacerbates the problem of feathering for prior-art shadow-mask-baseddirect patterning.

Embodiments in accordance with the present disclosure, however, employ acomposite structural layer from which a shadow-mask membrane is formed,where the composite layer has a plurality of constituent layers whosethicknesses and/or stresses are selected to effect:

-   -   i. compensation of a residual tensile stress in a first        constituent layer with a complimentary residual stress in a        second constituent layer, where the residual tensile stress of        the first constituent layer is higher than the fracture stress        of a membrane formed from the layer; and/or    -   ii. a membrane having a mechanical pre-bias that gives rise to a        bending moment directed opposite to the direction of gravity        when the shadow mask is installed in its intended deposition        system.

As discussed below, by selecting the thicknesses and residual stressesof the constituent layers of a structural layer from which a shadow-maskmembrane is formed, the membrane can be formed such that it has atendency to “bulge” in the direction opposite gravity when installed insystem 100. When oriented in a plane that enables the force of gravityto act in opposition to this tendency, the bulge is reduced and,preferably, substantially eliminated. The amount of mechanical pre-biasis typically based on the orientation of the shadow mask in the chamberduring deposition (normally known a-priori) such that when the shadowmask is installed in the deposition system, a substantially flatmembrane is realized.

FIG. 3 depicts a schematic drawing of a cross-sectional view of a shadowmask 108. Shadow mask 108 includes handle substrate 302, structurallayer 304, and apertures 126. It should be noted that FIG. 3 depictsshadow mask 108 without accounting for the effect of gravity on themembrane.

FIG. 4 depicts operations of a method for forming a mechanicallypre-biased shadow mask in accordance with the illustrative embodiment.Method 400 is described herein with continuing reference to FIG. 3, aswell as reference to FIGS. 5A-D. Method 400 begins with operation 401,wherein composite layer 304 is formed on handle substrate 302.

Handle substrate 302 is a conventional silicon wafer suitable for use inplanar processing. In some embodiments, substrate 302 comprises adifferent material suitable for use in planar processing. Materialssuitable for use in substrate 302 include, without limitation, siliconcompounds, compound semiconductors, other semiconductors, ceramics,composite materials, and the like.

Composite layer 304 is a structural layer comprising layers 308 and 310.Layers 308 and 310 are in intimate physical contact such that neitherlayer can move independently of the other.

In the depicted example, layer 308 is a layer of stoichiometric siliconnitride having a thickness of approximately 50 nm and a residualcompressive stress of approximately 1 GPa. Layer 308 is deposited on topsurface 306 of substrate 302 via low-pressure chemical vapor deposition(LPCVD). One skilled in the art will recognize that the “as-deposited”residual stress of a stoichiometric silicon nitride layer is based onthe conditions under which it is deposited, including the material ofthe substrate on which it is deposited, deposition temperature,deposition rate, chamber pressure, precursor gas selection, etc. For thepurposes of this Specification, including the appended claims, the term“as-deposited residual stress” is defined as the residual stress thatexists in a layer of material as a consequence of its formation on agiven substrate before any mechanical relaxation in the layer is enabled(neglecting wafer bow)—for example, by releasing a portion of the layerto form a membrane. In some embodiments, the stress of layer 308 isother than 1 GPa. In some embodiments, layer 308 is deposited via aconventional deposition process other than LPCVD, such as atomic-layerepitaxy, plasma-enhanced chemical vapor deposition (PECVD), sputtering,and the like. In some embodiments, layer 308 comprises a material otherthan stoichiometric silicon nitride, such as silicon-rich siliconnitride, silicon oxynitride, one or more metals, one or more polymers,etc.

Layer 310 is a layer of stoichiometric silicon dioxide having athickness of approximately one micron and a residual compressive stressof approximately 400 MPa. Layer 310 is formed on layer 308 by LPCVDusing a precursor gas of tetraethyl orthosilicate (TEOS). One skilled inthe art will recognize, after reading this Specification, however, thatlayer 310 can be formed in myriad conventional ways, such asspin-coating, atomic-layer deposition, PECVD, sputtering, and the like.As for layer 308, the specific value of the “as-deposited” residualstress in layer 310 is a function of its material, as well as theconditions under which it is deposited, including the material of thesubstrate on which it is deposited, deposition temperature, depositionrate, chamber pressure, precursor gas selection, etc. As a result, thestress of layer 310 can be other than 400 MPa without departing from thescope of the present disclosure. In some embodiments, layer 310comprises a material other than stoichiometric silicon dioxide, such asa different silicon oxide, silicon oxynitride, one or more metals, oneor more polymers, etc.

In the depicted example, the residual stress in layer 310 iscompressive, which compensates, at least partially, the high residualtensile stress in layer 308. As a result, the composite layer ischaracterized by an “effective” residual stress that is less than thefracture stress of layer 308. This enables a larger membrane withoutfracture, while still maintaining the high tension desirable in thestoichiometric silicon nitride layer to effect a membrane with lessgravity-induced sag.

In some embodiments, the layer structure of composite layer 104 isreversed such that layer 310 is disposed directly on substrate 302. Insuch embodiments, layer 310 can be formed by oxidizing the top surfaceof handle substrate 302 in conventional fashion.

Composite layer 304 is characterized by neutral plane 316, which is theplane in its thickness at which the residual stress in the material isequal to zero. Because the residual stress in layers 308 and 310 isunbalanced, the location of neutral plane 316 within the thickness ofthe composite layer is closer to substrate 302 than the central plane ofthe composite layer.

In some embodiments, the residual stress in layers 308 and 310 is of thesame type (i.e., tensile or compressive) but of different magnitudes.For example, in some embodiments, the residual stress in each of layers308 and 310 is compressive, but the residual compressive stress in layer308 is greater (or less) than the residual compressive stress in layer310. In similar fashion, in some embodiments, the residual stress ineach of layers 308 and 310 is tensile, but the tensile stress in layer308 is greater (or less) than the compressive stress in layer 310.

In the depicted example, the LPCVD deposition of layers 308 and 310results in deposition of their materials on back surface 322 ofsubstrate 302. The layers formed on back surface 322 collectively definemask layer 502. In some embodiments, mask layer 502 includes one or moredifferent materials that are suitable for protecting a portion ofsurface 322 during the formation of cavity 312. In some embodiments,mask layer 502 is formed independently of composite layer 304 in aseparate operation or set of operations.

FIG. 5A depicts a schematic drawing of a cross-sectional view of nascentshadow mask 108 after the formation of composite layer 304 on surface306 and mask layer 502 on surface 322.

At operation 402, apertures 126 are formed through the entire thicknessof composite layer 304.

Apertures 126 are through-holes formed in composite layer 304 viareactive-ion etching (RIE). In some embodiments, apertures 126 areformed in another conventional manner. Apertures 126 are sized andarranged based on the desired pattern of material to be deposited on atarget substrate. In some embodiments, apertures 126 are arranged tomatch the desired pattern of deposited material. In some embodiments,apertures 126 are arranged in a pattern that compensates for featheringacross the area of the substrate. Typically, apertures 126 are formedthrough composite layer 304 before the formation of cavity 312.

At operation 403, mask 324 is formed on the backside of substrate 302.

To form mask 324, layer 502 is patterned, typical via photolithographyand reactive-ion etching (or another conventional etching process) todefine opening 504, thereby exposing the central region of surface 322.

FIG. 5B depicts a schematic drawing of a cross-sectional view of nascentshadow mask 108 after the definition of apertures 126 and mask 324.

At operation 404, membrane 314 is defined by releasing a portion ofcomposite layer 304 from handle substrate 302 by forming cavity 312. Itshould be noted that, preferably, cavity 312 is not formed until afterthe deposition of all of the layers of composite layer 304. In someembodiments, cavity 312 is partially formed prior to the formation of atleast one of the constituent layers of composite membrane 314.

Cavity 312 is formed by removing the material in the center of handlesubstrate 302 while leaving the outer portion of substrate 302 asannular support frame 506, which extends around the perimeter ofcomposite layer 304. In the depicted example, cavity 312 is formed byremoving the exposed silicon region via a crystallographic dependentetch (e.g., ethylene diamine pyrocatechol (EDP), potassium hydroxide(KOH), hydrazine, etc.). In some embodiments, cavity 312 is formed viadeep reactive-ion etching, or other conventional process.

Upon release of membrane 314 from the handle substrate, each of layers308 and 310 partially relaxes to relieve internal strain (which is dueto its residual stress). In the depicted example, the free portion oflayer 308 attempts to contract (due to its residual tensile stress)while the free portion of layer 310 attempts to expand (due to itsresidual compressive stress). It should be noted that, without thepresence of tensilely stressed layer 308, the released portion of layer310 is mechanically bi-stable such that it is equally likely to buckleinward (i.e., toward substrate 302) or outward. The inclusion oftensilely stressed layer 310, however, causes the bulge to manifestoutward because that configuration represents the lower energy state forthe composite membrane. In other words, if the bulge were inward, thetensilely stressed layer would have higher strain due to its position onthe outer (i.e., larger) surface of the bulge. In contrast an outwardbulge locates the tensile layer on the inner (i.e., smaller) surface ofthe composite membrane.

In some embodiments, cavity 312 is formed such that a thin layer ofsilicon remains in contact with composite membrane 314, thereby defininga membrane that includes an additional layer of silicon.

FIG. 5C depicts a schematic drawing of a cross-sectional view of nascentshadow mask 108 after the formation of cavity 312. It should be notedthat this depiction of the shadow mask does not account for the effectof gravity-induced sag on the shadow mask membrane.

In the depicted, example, the stress configuration of layers 308 and 310locates neutral axis 316 of membrane 314 below the central plane ofcomposite layer 304 (i.e., the plane located at half its thickness),which gives rise to a bending moment 508 directed in the positive zdirection.

In some embodiments, the thicknesses and/or stresses of layers 308 and310 are selected to effect an inward bulge of membrane 314 (i.e., itbulges in the negative z-direction toward substrate 302).

It should be noted that, although the illustrative embodiment comprisesa composite layer having two layers of opposite stress, other layerconfigurations that effect a mechanical prebias in the composite layerare within the scope of the present disclosure.

Alternative layer configurations for composite membranes in accordancewith the present disclosure include: membranes having more than twolayers; membranes having multiple layers of the same stress, wherein atleast two of the layers have residual stresses of different magnitudes;membranes having layers whose thicknesses and residual stresses induce abulge directed along the direction from the compressively stressed layerto the tensilely stressed layer, and the like.

By choosing the thicknesses and internal stresses of layers 308 and 310appropriately, the force on the membrane due to residual-stress-inducedbending moment 508 can balance, or partially balance) the force due togravity-induced bending moment 510, which arises when the shadow mask isoriented such that gravity is directed in the negative z direction.

As a result, embodiments in accordance with the present disclosuremitigate gravity-induced sag that arises in prior-art shadow masks whenthey are mounted in a deposition chamber.

At operation 405, shadow mask 108 is located in system 100 such that theshadow mask is oriented in a horizontal position (i.e., in the x-yplane). As a result, angle θ is equal to 90° and residual-stress-inducedbending moment 508 and gravity-induced bending moment 510 are perfectlyopposed and balanced. As a result, the top surface of shadow mask 108 isflat and defines plane 120.

FIG. 5D depicts a schematic drawing of a cross-sectional view of nascentshadow mask 108 when oriented such that residual-stress-induced bendingmoment 508 and gravity-induced bending moment 510 are balanced.

It is to be understood that the disclosure teaches just some embodimentsin accordance with the present disclosure and that many variations caneasily be devised by those skilled in the art after reading thisdisclosure and that the scope of the invention is determined by thefollowing claims.

What is claimed is:
 1. A shadow mask that includes: a substrate thatdefines a first plane, the substrate including a cavity; and a compositelayer that is disposed on the substrate, the composite layer having afirst region that is disposed over the cavity and defines a membrane,wherein the first region includes an aperture pattern comprising atleast one aperture, and wherein the composite layer comprises aplurality of layers that includes (1) a first layer having a firstthickness and a first residual stress and (2) a second layer having asecond thickness and a second residual stress; wherein the firstthickness, second thickness, first residual stress, and second residualstress collectively give rise to a first bending moment in the membrane,the first bending moment being directed along a direction that issubstantially normal to the first plane.
 2. The shadow mask of claim 1wherein the first layer is characterized by a fracture stress, andwherein the first residual stress is greater than the fracture stress.3. The shadow mask of claim 1 wherein the first material isstoichiometric silicon nitride.
 4. The shadow mask of claim 3 whereinthe second material is stoichiometric silicon dioxide.
 5. The shadowmask of claim 1 wherein the first residual stress is tensile and thesecond residual stress is compressive.
 6. The shadow mask of claim 1wherein each of the first residual stress and second residual stress iscompressive.
 7. The shadow mask of claim 1 wherein each of the firstresidual stress and second residual stress is tensile.
 8. The shadowmask of claim 1 wherein the plurality of layers includes a third layer.9. The shadow mask of claim 1 wherein the magnitude of the first bendingmoment is substantially equal to a second bending moment induced inmembrane by the force of gravity on the membrane.
 10. The shadow mask ofclaim 1 wherein the first bending moment has a magnitude sufficient toinduce a curvature of the membrane.
 11. A method comprising forming ashadow mask, wherein the shadow mask is formed by operations thatinclude: providing a substrate that defines a first plane; forming acomposite layer on the substrate, the composite layer including: a firstlayer comprising a first material, the first layer having a firstthickness and a first residual stress; and a second layer comprising asecond material, the second layer having a second thickness and a secondresidual stress; defining an aperture pattern in a first region of thecomposite layer, wherein the aperture pattern includes at least oneaperture, and wherein the at least one aperture extends completelythrough the composite layer; and forming a cavity in the substrate,wherein the cavity is formed after the formation of the composite layer,and wherein the formation of the cavity releases the first region of thecomposite layer to define a membrane; wherein the first thickness,second thickness, first residual stress, and second residual stress areselected such that the membrane has a first bending moment that isdirected along a direction that is substantially normal to the firstplane.
 12. The method of claim 11 further comprising forming the firstlayer such that it is characterized by a fracture stress that is lessthan the first residual stress.
 13. The method of claim 11 furthercomprising forming the first layer such that the first material isstoichiometric silicon nitride.
 14. The method of claim 13 furthercomprising forming the second layer such that the second material isstoichiometric silicon dioxide.
 15. The method of claim 11 wherein thecomposite layer is formed such that the first residual stress is tensileand the second residual stress is compressive.
 16. The method of claim11 wherein the composite layer is formed such that the first residualstress and the second residual stress are compressive.
 17. The method ofclaim 11 wherein the composite layer is formed such that the firstresidual stress and the second residual stress are tensile.
 18. Themethod of claim 11 wherein the composite layer is formed such that theplurality of layers includes a third layer.
 19. The method of claim 11wherein the composite layer is formed such that the magnitude of thefirst bending moment is substantially equal to a second bending momentthat is based on the force of gravity on the composite membrane.
 20. Themethod of claim 11 wherein the first thickness, second thickness, firstresidual stress, and second residual stress are selected such that thefirst bending moment has a magnitude sufficient to induce a curvature ofthe membrane.
 21. The method of claim 11 wherein the second layer isformed on the first layer while the first layer is characterized by itsas-deposited residual stress.